Saurabh@educational

Changes around us Q.5. Give some examples of reversible and irreversible changes involving in same materials. Ans.  S.No. Example Reversible Irreversible 1. Folding a paper and cutting the paper Folding a paper Cutting the paper 2. Rolling and baking of roti Rolling of roti Baking of roti 3. Shaping wet clay into clay pot and baking a clay pot Shaping wet clay into clay pot Baking a clay pot 4. Inflating and bursting a balloon Inflating a balloon Bursting a balloon 5. Melting of wax and burning of wax Melting of wax Burning of wax Q.6. What do we mean by terms 'Expansion' and 'Contraction'?  Ans.  The increase in size on heating is called expansion while t he decrease in size on cooling is called contraction. For example: In thermometers, mercury(which is a liquid metal at room temperature) is used. When the bulb o...

Changes around us Q.1. What do you understand by the term 'Change'? Ans. When one or more properties of a thing become different. We say it has changed or a change has taken place. For example: Formation of curd from milk Cooking of food Melting of ice Growing of hairs and nails Leaves fall from trees, change colour and dry out Q.2. Give some examples of beneficial changes and harmful changes. Ans.  some beneficial changes can be : The ripening of fruits Cooking of the food Formation of curd from milk         some harmful changes can be : The spoiling of food Rusting of iron Rubbing of soles of shoes Q.3. Do changes occur on their own? Give one example. Ans.  It appears that many changes are taking place around us on their own but changes do not occur on their own. There is always a ' cause'  which brings a change. For example: heat is the cause of change in the state...

FRICTION Q.11. Define drag. Ans.   The frictional force exerted by fluids (gases and liquids) is called drag.  Q.12. What are the factors on which frictional force on an object in a fluid depends? Ans. The frictional force on an object in a fluid depends on its  speed with respect to the fluid shape of the object and the nature of the fluid.  Q.13. Explain why objects moving in fluids must have special shapes. Ans. Fluids exert force of friction on objects in motion through them. So, when objects move through fluids, they have to overcome friction acting on them. In this process they lose energy. Efforts are, therefore, made to minimize friction. As frictional force on an object in a fluid depends on the shape of the object. So, objects are given special shapes that are streamlined shape to minimize friction. Q.14. Iqbal has to push a lighter box and Seema has to push a similar heavier b...

Separation of Substances Q.11. Write a short note on each:            Evaporation and condensation Ans.  Evaporation Evaporation is the process in which a liquid gets converted into its vapour.  Evaporation can be used to separate a solid dissolved in a liquid. The process of evaporation takes place continuously wherever water is present. For example:  Steam rises from wet clothes while they are ironed. Blackboard dries up after wiping it. The process of evaporation is used to separate a mixture of water and salt. Condensation The process of conversion of liquid vapour into its liquid form is called condensation. For example: Water drops appear on the outer surface of a glass containing cold water.  We see water drops condensed under a plate that has been used to cover a vessel containing milk that has just been boiled. Q.12. How do we get salt from sea water? Ans.  Sea wat...

Heat Q.  What are sea breeze and land breeze? Ans. The people living in the coastal areas experience an interesting phenomenon. Sea breeze : During the daytime, the land gets heated faster than the water . The air over the land becomes hotter and r i ses up to create a low pressure of air. The colder air from t he sea which is at higher pressure r ushes towards the land to bala nce the pressure. The warm air from the land moves towards the sea to complete the cycle. This causes air to move from sea towards the land and i s called sea breeze.             To receive the cooler sea breeze, the windows of the houses in coastal areas are made to face the sea .                  Land breeze : During night, the land cools up quickly than water. On cooling, a low pressure of air is created on sea (because sea is still hotter than land) while a high pressure of air is created on land. So...